It is known that the volumetric flow rate of a fluid passing through a rectangular metering gap is governed by, and is inversely proportional to, flow resistance, where the flow resistance is comprehended by the formula
      Δ    ⁢                  ⁢    P    =            12      ⁢                          ⁢      η      ⁢                          ⁢      LQ                      WH        3            ⁢              F        p            where P is Pressure, η is the fluid viscosity, Q is the volumetric flow rate, W is the width of the flow channel transverse to the main direction of fluid outflow, L is the length of the transverse metering channel in the main outflow direction, H is the transverse metering channel gap, and Fp is the geometric shape factor. It can be appreciated from this formula that volumetric flow rate is responsive to flow resistance and is particularly sensitive to the metering gap.
To provide regulation of the transverse mass flow distribution through a slot die, a transverse flow metering channel (commonly called a preland channel), is typically positioned between and in fluid communication with a transverse flow-providing manifold and an exit channel. The transverse flow metering channel provides regulation of the transverse distribution of a flow stream by regulating the resistance to flow through a combination of gap and gap length geometries defining the transverse flow metering gap and channel in accordance with the governing principles of the preceding formula. Depending upon process objectives, a transverse flow metering channel may be used to provide a generally uniform, or a non-uniform, casting or coating thickness.
A conventional single cavity slot die as generally illustrated in FIG. 1, may include a body shim 1 disposed between two die bodies 2,3 to provide selection of a lip gap complementary to the processability and application of the particular fluid to be processed. A slot die flow passageway includes a transverse flow-providing manifold 4, a flow metering section 5 (preland channel) that provides a transverse flow metering gap, and an exit channel that provides a metering function and that includes an exit orifice 6 (the lip gap). Metering of the fluid through the lip gap is critical to the particular casting or coating process employed; and as a result, there is a need to select and establish an appropriate lip gap based on considerations including the particular fluid to be processed and/or the downstream process. To this end, a body shim may be beneficially selected to provide a predetermined lip gap, and as illustrated, is usefully disposed between the two die bodies and generally parallel to the Z-axis of an X-Y-Z coordinate system (see FIG. 5 for all three axes of the X-Y-Z coordinate system).
Without a body shim, the lip gap would be relatively smaller than illustrated (compare for example, to the lip gap of the slot die of FIG. 2). Body shims having a variety of thicknesses are available, and can be substituted for one another, to select a variety of predetermined lip gaps. A series of body assembly bolts A (only one shown) fasten die bodies 2,3 together and extend through mechanical clearance holes C (only one shown) in die body 2. A body shim can be replaced by disassembling the slot die to provide access to the body shim, substituting one body shim for another, and re-assembling the die bodies back together.
Hypersensitivity of volumetric flow rate to metering gap changes in slot die processes, whether single cavity or multi-cavity slot dies, is known to be attributed to its inverse proportionality to flow resistance, as supported by the foregoing formula. As illustrated by U.S. Pat. Nos. 4,372,739 to Vetter, 4,695,236 to Predohl et al, 4,708,618 to Reifenhauser et al, 5,066,435 to Lorenz et al, and 5,147,195 to inventor Peter F. Cloeren, apparatus used for extruding thermoplastic resins may use a slidably adjustable restrictor bar to provide metering gap adjustment. Mechanical clearance allows movement of a restrictor bar, by a bolt that passes through a die body.
It is typical to change the exit orifice width to accommodate different product width requirements. Conventional external deckles are not suitable for slot die casting or coating processes, due to the close proximity of the die exit slot to the casting or coating surface, typically in the range of 50 to 3000 μm. Thus, a casting fluid or coating fluid is generally delivered from a slot die in close proximity to a casting or coating surface. Nor are slot dies suitable for conventional internal deckles, as generally illustrated in Cloeren U.S. Pat. Nos. 5,451,357 and 5,505,609, due to the relatively small metering gaps used. Accordingly, in a slot die, a deckle shim (see FIGS. 10 and 11 for illustrative prior art deckle shims) may be conveniently disposed between die bodies 2,3 instead of body shim 1 (see FIG. 1) to establish a predetermined lip gap (in which case it functions in part as a body shim) and a predetermined exit orifice width.
However, with reference again to the slot die of FIG. 1, a problem with the prior art use of a body shim or a deckle shim to establish a predetermined lip gap is that the shim also simultaneously changes the transverse flow metering gap of flow metering section 5. As can be appreciated from the very small transverse flow metering gaps customarily used and the resulting volumetric flow rate hypersensitivity, any change in the transverse flow metering gap can adversely affect the desired transverse metering of the fluid flow provided by metering section 5. Different approaches such as a two stage preland channel as exemplified by U.S. Pat. No. 5,256,052 issued to inventor Peter F. Cloeren, and a coat-hanger shaped preland channel, have been applied in the prior art slot die of FIG. 1 but found to be subject to volumetric flow rate hypersensitivity, and accordingly to be ineffective in sufficiently regulating the transverse flow through flow metering section 5 when a body shim or deckle shim of a different thickness is selected to establish a different predetermined lip gap.
Furthermore, in slot die casting or coating processes, adjustment of the transverse flow metering gap independent of changing the lip gap can be beneficial or necessary to accommodate process changes such as different flow rates, different fluid viscosities, and other process objectives.
With reference now to the slot die of FIG. 2, it is also known in the prior art to fit die bodies 2,3 with removably mounted lip inserts 7 to form exit orifice 6. Lip inserts are beneficial, for example, for general wear and tear purposes. Although not shown in FIG. 2, it is recognized that a lip shim can be inserted between a surface 8 of the removably mounted lip insert and the respective die body surface to establish the lip gap. However, a drawback for certain slot die applications, is a resulting interruption of the flow surface of the exit channel at the junction of the lip insert and the respective die body.
With continued reference to the slot die of FIG. 2, it is known for certain process applications to use an exit orifice in which one lip extends beyond the other lip. Die bodies 2,3 are normally non-adjustable relative to one another as a result of being fastened together by a series of body assembly bolts A (only one shown). However, loosening body assembly bolts A allows positioning of die bodies 2,3 relative to one another. To also allow the relative positioning, it is known for die body 2 to be provided with bolt clearance apertures D (only one shown), appropriately sized to provide for Z-axis relative movement of die bodies 2,3. It is also known for die body 3 to include a backing structure B, and for a backing shim 9 of an appropriate thickness to be removably disposed between backing structure B and a rear wall R of die body 2. The backing structure may, as illustrated, be an integral part of die body 3, or may be removably attached. For clarity of illustration, body bolt A is shown in a rearward position relative to clearance aperture D.
Accordingly, there continues to be a need for an improved slot die and methodology that provide for transverse flow metering gap adjustment independent of changing other flow channel metering gaps. Beneficially, an improved slot die would allow adjustment of a transverse flow metering gap independent and apart from changing the lip gap. Beneficially, an improved slot die would also allow selecting a different lip gap by the use of body shims of different thicknesses or otherwise, independent and apart from changing the transverse flow metering gap. Advantageously, an improved slot die would also allow selecting a different exit orifice width or dividing an outflowing stream into two or more streams by the use of deckle shims, independent and apart from changing the transverse flow metering gap.